Concept Guide

NOTE: The insertion of the tag header into the Ethernet frame increases the size of the frame to more than the 1,518 bytes
as specified in the IEEE 802.3 standard. Some devices that are not compliant with IEEE 802.3 may not support the larger
frame size.
Information contained in the tag header allows the system to prioritize traffic and to forward information to ports associated
with a specific VLAN ID. Tagged interfaces can belong to multiple VLANs, while untagged interfaces can belong only to one
VLAN.
Configuration Task List
This section contains the following VLAN configuration tasks.
● Creating a Port-Based VLAN (mandatory)
● Assigning Interfaces to a VLAN (optional)
● Assigning an IP Address to a VLAN (optional)
● Enabling Null VLAN as the Default VLAN
Enabling Null VLAN as the Default VLAN
In a Carrier Ethernet for Metro Service environment, service providers who perform frequent reconfigurations for customers
with changing requirements occasionally enable multiple interfaces, each connected to a different customer, before the
interfaces are fully configured.
This presents a vulnerability because both interfaces are initially placed in the native VLAN, VLAN 1, and for that period
customers are able to access each other's networks. The system has a Null VLAN to eliminate this vulnerability. When you
enable the Null VLAN, all ports are placed into it by default, so even if you activate the physical ports of multiple customers, no
traffic is allowed to traverse the links until each port is place in another VLAN.
To enable Null VLAN, use the following command.
● Disable the default VLAN, so that all ports belong to the Null VLAN until configured as a member of another VLAN.
CONFIGURATION mode
default-vlan disable
Default: the default VLAN is enabled (no default-vlan disable).
Assigning an IP Address to a VLAN
VLANs are a Layer 2 feature. For two physical interfaces on different VLANs to communicate, you must assign an IP address to
the VLANs to route traffic between the two interfaces.
The shutdown command in INTERFACE mode does not affect Layer 2 traffic on the interface; the shutdown command only
prevents Layer 3 traffic from traversing over the interface.
NOTE:
You cannot assign an IP address to the Default VLAN (VLAN 1). To assign another VLAN ID to the Default VLAN,
use the default vlan-id vlan-id command.
You can place VLANs and other logical interfaces in Layer 3 mode to receive and send routed traffic. For more information, refer
to Bulk Configuration.
To assign an IP address, use the following command.
● Configure an IP address and mask on the interface.
INTERFACE mode
ip address ip-address mask [secondary]
○ ip-address mask — Enter an address in dotted-decimal format (A.B.C.D) and the mask must be in slash format
(/24).
○ secondary — This is the interface’s backup IP address. You can configure up to eight secondary IP addresses.
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Virtual LANs (VLANs)